Field
Embodiments described herein relate generally to an analog switch and a multiplexer.
Background Art
Automobiles such as hybrid cars and electric vehicles (EV) that use multiple secondary batteries connected in series or power storage systems need to monitor the amount of electricity remaining in the battery cells and precisely measure the voltage of each battery cell for cell abnormality detection. According to prior art, a multi-channel analog switch that connects an arbitrary cell to an A/D converter is used for such measurement.
There is a high-voltage analog switch that is used in a secondary battery system that has a plurality of cells connected in series with each other and provides electric power at a voltage of several tens volts or higher. The FET of the high-voltage analog switch is required to withstand a voltage of several tens volts or higher at least between the drain and the source.
However, the FET may only be able to withstand a voltage of several to a dozen volts between the gate and the source. In that case, a current is passed to a resistor connected between the gate and the source of the FET to achieve a controlled voltage falling within the withstand voltage range between the gate and the source.
However, part of the current flowing through the resistor flows to the FET to cause a voltage drop, which leads to a voltage difference between the input and the output of the analog switch. For precise measurement, the difference needs to be cancelled.
A method of cancelling the difference involves connecting a current source that derives, from the source side, a current that has a strength equal to the strength of the current passed to the gate side of the resistor.
However, if the voltage of the connected cell is the minimum or maximum voltage, it is difficult to make the currents agree with each other until the difference is completely cancelled.
The voltage drop caused by the current flowing to the FET and the on-resistance of the FET produces a difference between the voltage of the input signal and the voltage of the output signal of the analog switch.